Light receiving element and light receiving device incorporating circuit and optical disc drive

ABSTRACT

A light receiving device includes a P type diffusion layer ( 101 ), a P type semiconductor layer ( 102 ), an N type diffusion layer ( 103 ) serving as a light receiving part, and a light transmitting film ( 104 ), all formed on a p type silicon substrate ( 100 ). The N type diffusion layer ( 103 ) has a thickness of 0.8 μm to 1.0 μm which is larger than an absorption length of incident light having wavelength of 400 nm, and such a concentration profile that a impurity concentration is not higher than 1E19 cm −3  on a surface and has a peak in a vicinity of the surface. Since recombination of carriers generated by the incident light is prevented in the vicinity of the surface of the N type diffusion layer ( 103 ), sensitivity of the light receiving device is enhanced and response speed is increased by the low-resistance N type diffusion layer ( 103 ) having a larger junction depth.

This Nonprovisional application claims priority under 35 U.S.C. §119(a)on Patent Application No. P2001-394221 filed in Japan on Dec. 26, 2001,the entire contents of which are hereby incorporated by reference.

TECHNICAL FIELD

The present invention relates to a light receiving device, a lightreceiving unit incorporating a circuit, and an optical disk drive.

BACKGROUND ART

Conventionally, optical disk drives for optical disks such as CDs(Compact Discs) and DVDs (Digital Versatile Disks) are provided with anoptical pickup. The optical pickup includes a semiconductor laser devicefor emitting a light beam to be radiated to an optical disk and a lightreceiving device for receiving a reflected light beam radiated to andreflected by the optical disk. In recent years, higher-density DVDs arebeing vigorously developed, which demands for processing large-volumedata such as dynamic images and for higher read rates such as 12X. Sincean amount of a data storage capacity of the optical disk such as theabove-stated DVDs is inversely proportional to the square volume of awavelength of radiated light, an attempt to shorten the wavelength ofthe light emitted from the semiconductor laser device in the pickupsystem is being pursued.

In the above pickup system, with the shorter wavelength of the lightemitted from the semiconductor laser device, the light receiving deviceis required to convert incident light to an electronic signal at highefficiency. In other words, it is necessary to enhance the sensitivityof the light receiving device to the incident light. The sensitivity ofthe light receiving device is indicated by the following formula:$\begin{matrix}{{Sensitivity} = {\frac{photocurrent}{{Intensity}{\quad\quad}{of}\quad{incident}\quad{light}} = {\frac{I_{ph}}{P_{i0}} = {\frac{q}{h_{c}/\lambda}\eta}}}} & \left( {1 - R} \right)\end{matrix}$wherein “q” denotes an elementary quantity of electric charge, “h”denotes a Planck's constant, “λ” denotes a wavelength of incident light,“c” denotes a light speed, “η” denotes a quantum yield, and “R” denotesa surface reflectance of the device that is a ratio of reflected lighton the surface of the light receiving device to incident light.

In the light receiving device, minority carriers generated by incidentlight can be taken out as electric current at high efficiency if thecarriers are generated in the vicinity of a PN junction part, which isformed for forming electric fields at a specified depth position fromthe light incidence surface of the device. Herein, if light with theintensity P_(i0) is incident to a medium, the light intensity P_(i)(x)at a depth x in the medium from the incidence surface is obtained asfollows:−dP _(i)(x)=α₀ P _(i)(x)dx  (1)−dP _(i)(x)/dx=−α ₀ P _(i)(x)  (2)therefore,Pi(x)=P _(i0) exp(−α₀ x)  (3)where “α₀” is an absorption coefficient, which is a physical constantthat varies depending on both the medium and wavelength of light. Lightradiated to the surface of the medium such as semiconductors comes intothe medium while being absorbed thereby, and its light intensity in themedium is exponentially decreased depending on the depth from thesurface as shown in the formula (3). Light with a wavelength havinglarger absorption coefficient α₀ is absorbed by the medium at closerposition to the surface, resulting in generation of carriers. A distanceL_(a) between the surface of the medium from which light is incident anda point in the depth direction of the medium at which the light arrivesis defined as below based on the formula (2):α₀ Pi ₀ L _(a)=∫^(∞) ₀α₀ P _(i0) exp(−α₀ x)dxhence, La=1/α₀Herein, the value L_(a) that is defined as the inverse number of anabsorption coefficient α₀ is referred to as an absorption length, andthe intensity of incident light at this position is expressed asexp(−1). For example, red incident light with a wavelength of about 600nm has an absorption coefficient α₀ of about 3000 cm⁻¹ and an absorptionlength of 3 μm, while blue-violet incident light with a wavelength ofabout 400 nm has a considerably large absorption coefficient α₀ of about50000 cm⁻¹, and a small absorption length of 0.2 μm.

This indicates that the light receiving device for receiving shortwavelength light needs to have a PN junction provided at a depthposition smaller than the absorption length of the incident light inorder to gain a sufficient sensitivity.

FIG. 14 is a view showing a conventional light receiving device (seeJapanese unexamined patent application No. H09-237912). This lightreceiving device is composed of a low-resistance N type diffusion layer501 and an N type semiconductor layer 502, each formed on a P typesemiconductor substrate 500. On the surface of the N type semiconductorlayer 502, there is formed a first P type diffusion layer 503functioning as a light receiving part, by which a PN junction isobtained. Reference numeral 504 denotes a second P type diffusion layerwith high concentration to reduce the resistance of the P type diffusionlayer 503. Reference numeral 505 denotes an N type high-concentrationdiffusion layer, which is in contact with the low-resistance N typediffusion layer 501. Reference numeral 506 is an insulating film. The PNjunction is formed at an extremely shallow position of 0.01 to 0.2 μm sothat the concentration of the first P type diffusion layer 503 is from1E16 cm⁻³ to 1E20 cm⁻³ and that a junction depth is smaller than theabsorption length of the wavelength of received light. Thus, thesensitivity, particularly to light with a wavelength of not larger than500 nm, is enhanced.

However, the conventional light receiving device has a problem that thejunction depth is so small that the response speed is declined. Forexample, with the junction depth of less than 0.2 μm, the resistance ishigher than that with the junction depth of 1.0 μm by approx. {fraction(1/10)}, resulting in drastic deterioration of the response speed. Ifimpurity concentration on the surface part of the light receiving deviceis increased to prevent rise of the resistance, recombination ofcarriers on the surface part becomes outstanding, thereby causingreduced sensitivity.

If the junction depth of the light receiving device is increased toprevent decline of the response speed, and light received by the lightreceiving device has short wavelength, then most part of the carriersare absorbed by the surface part of the light receiving device, causingdeteriorated sensitivity. Further, if a high-concentration diffusionlayer is additionally provided to reduce resistance as with the case ofthe conventional light receiving device, a light receiving area isenlarged and so the capacitance is increased, thereby causingdeteriorated response speed. More specifically, increase of speed andenhancement of the sensitivity of the light receiving device are intrade-off relation, and particularly in the case where light with shortwavelength is received for an attempt of further speedup of opticaldisks, achieving both increased speed and enhanced sensitivity isextremely difficult.

Accordingly, it is a primary object of the present invention to providea light receiving device that achieves both higher speed and highersensitivity.

DISCLOSURE OF THE INVENTION

In order to achieve the above object, the present invention provides alight receiving device comprising:

-   -   a first conductivity type semiconductor layer; and    -   a second conductivity type semiconductor layer on the first        conductivity type semiconductor layer, wherein    -   a thickness of the second conductivity type semiconductor layer        is larger than an absorption length of light incident to the        second conductivity type semiconductor layer, and    -   the second conductivity type semiconductor layer has impurity        concentration of not smaller than 1E17 cm⁻³ and not larger than        1E19 cm⁻³ in a vicinity of a surface of the second conductivity        type semiconductor layer.

According to the above configuration, although the thickness of thesecond conductivity type semiconductor layer is larger than theabsorption length of incident light to the semiconductor layer, and ajunction between the first conductivity type semiconductor layer and thesecond conductivity type semiconductor layer is placed at a relativelydeep position, the impurity concentration in the vicinity of the surfaceof the second conductivity type semiconductor layer is not smaller than1E17 cm⁻³ and not larger than 1E19 cm³. Consequently, in the vicinity ofthe surface of the second conductivity type semiconductor layer,recombination of carriers is effectively reduced, resulting in enhancedsensitivity of the light receiving device. Here, if the impurityconcentration in the vicinity of the surface of the second conductivitytype semiconductor layer is smaller than 1E17 cm⁻³, the resistance ofthe semiconductor layer is increased and therefore the response of thelight receiving device is deteriorated. If the impurity concentration inthe vicinity of the surface of the second conductivity typesemiconductor layer is larger than 1E19 cm⁻³, recombination of carriersin the vicinity of the surface of the semiconductor layer is increased,thereby causing deterioration of the sensitivity of the light receivingdevice.

Moreover, since the thickness of the second conductivity typesemiconductor layer is larger than the absorption length of incidentlight to the semiconductor layer, the resistance is lower than that inthe case of a conventional semiconductor layer whose thickness issmaller than the absorption length of incident light, so that the lightreceiving device is able to achieve higher response speed than that ofconventional one. Therefore, the light receiving device makes itpossible to obtain high performance by achieving both enhancedsensitivity and increased response speed.

The light receiving device comprising a first conductivity typesemiconductor layer and a second conductivity type semiconductor layeron the first conductivity type semiconductor layer herein refers to anyform of light receiving device such as those having second conductivitytype impurity diffused over a surface part of a first conductivity typesemiconductor layer to form a second conductivity type semiconductorlayer, and those having a second conductivity type semiconductor layerlaminated on a first conductivity type semiconductor layer.

The light receiving device of the present invention allows effectiveenhancement of sensitivity and response speed particularly whenreceiving red light with a wavelength of about 600 nm or less. Whenreceiving the red light with a wavelength of about 600 nm or less, theconventional light receiving device could not achieve both enhancedsensitivity and increased response speed even if a junction depth isdecreased to improve the sensitivity and impurity concentration isincreased to increase the response speed.

The inventor of the present invention found out through variousexperiments that enhanced sensitivity is achievable by controlling anin-depth profile of impurity concentration even if a junction is formedat a deep position contrary to the case of the conventional lightreceiving device, and hence invented the present invention based on thisfounding.

Furthermore, a light receiving device of the present inventioncomprises:

-   -   a first conductivity type semiconductor layer; and    -   a second conductivity type semiconductor layer on the first        conductivity type semiconductor layer, wherein    -   a thickness of the second conductivity type semiconductor layer        is larger than an absorption length of light incident to the        second conductivity type semiconductor layer, and    -   the second conductivity type semiconductor layer has impurity        concentration of not smaller than 1E17 cm⁻³ and not larger than        1E19 cm⁻³ at a position which is distant form a surface of the        second conductivity type semiconductor layer in a thickness        direction at a distance almost equal to the absorption length of        the light.

According to the above configuration, the impurity concentration of thesecond conductivity type semiconductor layer is not smaller than 1E17cm⁻³ and not larger than 1E19 cm ⁻³ at the position which is distantform the surface of the second conductivity type semiconductor layer inthe thickness direction at the distance almost equal to the absorptionlength of the light. Thus recombination of carriers generated in thevicinity of the surface of this second conductivity type semiconductorlayer is effectively prevented, which enhances the sensitivity of thelight receiving device. Therefore, even with the thickness of the secondconductivity type semiconductor layer being larger than the absorptionlength of the light, excellent sensitivity is achieved while responsespeed can be increased.

Herein, if the impurity concentration of the second conductivity typesemiconductor layer is smaller than 1E17 cm⁻³ at the position which isdistant form the surface of the second conductivity type semiconductorlayer in the depth direction at the distance almost equal to theabsorption length of the light, then the resistance of the semiconductorlayer is increased and response of the light receiving device isdeteriorated. If the impurity concentration of the second conductivitytype semiconductor layer is larger than 1E19 cm⁻³ at the position whichis distant form the surface of the second conductivity typesemiconductor layer in the thickness direction at the distance almostequal to the absorption length of the light, then recombination ofcarriers at the position with large impurity concentration is increasedand the sensitivity of the light receiving device is degraded.

In one embodiment, the second conductivity type semiconductor layer hasa peak impurity concentration on the surface.

According to the above-mentioned embodiment, the second conductivitytype semiconductor layer has the peak impurity concentration on thesurface, so that carriers generated by light incident to the secondconductivity type semiconductor layer are effectively prevented fromrecombining in the vicinity of the surface of this semiconductor layer.Therefore, most of the carriers generated by the incident light canreach the junction part, as a result of which the light receiving devicecan achieve sufficient sensitivity.

Furthermore, a light receiving unit incorporating a circuit of thepresent invention comprises:

-   -   the aforementioned light receiving device; and    -   a signal processing circuit for processing a signal from the        light receiving device, wherein    -   the light receiving device and the signal processing circuit are        formed on an identical substrate.

According to the above configuration, the light receiving device and thesignal processing circuit are formed monolithically so that a small-sizelight receiving unit having excellent sensitivity and high responsespeed is attained.

Furthermore, an optical disk drive of the present invention comprisesthe aforementioned light receiving device or the aforementioned lightreceiving unit incorporating a circuit.

According to the above configuration, the light receiving device or thelight receiving unit incorporating a circuit having excellentsensitivity and high response speed is used, thus, an optical disk driveparticularly suitable for read and write access to a mass storageoptical disk with use of light with a short wavelength is provided.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1A is a plan view showing a light receiving device in a firstembodiment of the present invention, while FIG. 1B is a cross sectionalview taken along the line and shown by arrows A-A′ in FIG. 1A;

FIG. 2 is a graph showing the relation between impurity concentration inthe vicinity of the surface of a light receiving part and sensitivity ofthe light receiving device in the first embodiment;

FIG. 3 is a graph showing the relation between impurity concentration inthe vicinity of the surface of the light receiving part and cathoderesistance of the light receiving part of the light receiving device inthe first embodiment;

FIG. 4 is a graph showing the relation between the cathode resistance ofthe light receiving part and response speed of the light receivingdevice in the first embodiment;

FIG. 5 is a graph showing an impurity concentration profile of the lightreceiving part of the light receiving device in the first embodiment;

FIG. 6 is a plan view showing a light receiving device in the firstembodiment provided with a plurality of cathode electrodes 108;

FIG. 7 is a graph showing an impurity concentration profile of the lightreceiving part of the light receiving device in the second embodiment;

FIG. 8 is a graph showing the relation between impurity concentration inthe vicinity of the surface of a light receiving part and sensitivity ofthe light receiving device in the second embodiment;

FIG. 9 is a graph showing the relation between impurity concentration inthe vicinity of the surface of the light receiving part and cathoderesistance of the light receiving part of the light receiving device inthe second embodiment;

FIG. 10 is a cross sectional view showing a light receiving device in athird embodiment of the present invention;

FIG. 11 is a graph showing an impurity concentration profile of thelight receiving part of the light receiving device in the thirdembodiment;

FIG. 12 is a cross sectional view showing a light receiving unitincorporating a circuit in a fourth embodiment of the present invention;

FIG. 13 is a view showing an optical disk drive in a fifth embodiment ofthe present invention; and

FIG. 14 is a cross sectional view showing a conventional light receivingdevice;

BEST MODE FOR CARRYING OUT THE INVENTION

Embodiments of the invention will now be described in detail withreference to the accompanying drawings.

(FIRST EMBODIMENT)

FIG. 1A is a plan view showing a light receiving device in a firstembodiment of the present invention, while FIG. 1B is a cross sectionalview taken along the line and shown by arrows A-A′ in FIG. 1A. It isnoted that in the present embodiment, multilevel interconnections andinterlayer films formed after the step for processing metalinterconnections are deleted.

As shown in FIG. 1B, the light receiving device has a P type diffusionlayer 101 with impurity concentration of about 1E18 cm⁻³ and a thicknessof about 1 μm on a P type silicon substrate 100, and on top surface ofthe P type diffusion layer 101, has a P type semiconductor layer 102 asa first conductivity type semiconductor layer with impurityconcentration of 1E13 cm⁻³ to 1E15 cm⁻³ and a thickness of about 10 μmto 20 μm. In the vicinity of the surface of the P type semiconductorlayer 102, there is formed an N type diffusion layer (cathode) 103 as asecond conductivity type semiconductor layer serving as a lightreceiving part. An impurity forming this N type diffusion layer 103 is apentavalent impurity such as P (phosphorous). On the N type diffusionlayer 103, there is disposed a light transmitting film 104 as anantireflection film, which is composed of a silicon oxide 105 and asilicon nitride 106. The film thickness of the silicon oxide 105 and thesilicon nitride 106 is so set as to have the lowest reflectance to lightthat comes incident to the light receiving device. More specifically, ifthe wavelength of the incident light to the light receiving device is400 nm, the thickness of the silicon oxide 105 is set at 10 nm to 30 nmwhile the thickness of the silicon nitride 106 is set at 20 nm to 50 nm.It is to be noted that without being limited to two layers, the lighttransmitting film 104 may be composed of a singularity of layer or aplurality of layers not smaller than 3 layers. Further, without beinglimited to the silicon oxide or silicon nitride, the light transmittingfilm 104 may be composed of any material.

Reference numeral 107 denotes a P type diffusion layer for pulling outan anode electrode, which is formed so as to extend from the top surfaceof the P type semiconductor layer 102 to the P type diffusion layer 101.The P type diffusion layer 107 has impurity concentration of around 5E19cm⁻³ to 1E21 cm⁻³ in the vicinity of the surface. Reference numeral 108denotes an electrode pulled out from a cathode, i.e., the N typediffusion layer 103.

The layer thickness, i.e., the junction depth of the PN junction as wellas surface impurity concentration of the N type diffusion layer 103 areso set as to obtain excellent sensitivity and response speed of thislight receiving device. FIG. 2 is a graph showing changes in sensitivityof a light receiving device having a junction depth of 0.7 μm to 1.2 μmto incident light with a wavelength of 400 nm when surface concentrationof impurity in the cathode is changed. The horizontal axis of FIG. 2represents the surface concentration (cm⁻³) of cathode in the lightreceiving part while the vertical axis represents sensitivity (A/W). Asshown in FIG. 2, when the light received by the light receiving devicehas a wavelength of about 400 nm, keeping the surface impurityconcentration of the N type diffusion layer 103 to not larger than 1E19cm⁻³ makes it possible to obtain excellent sensitivity with 90% or morequantum efficiency even though the junction depth is larger than theabsorption length of incident light. Here, in an impurity concentrationprofile of the N type diffusion layer 103, if a peak of the impurityconcentration is present at a relatively deep position near thejunction, then recombination of carriers occurs in the vicinity of thesurface of the N type diffusion layer 103. Consequently, the percentageof the carriers that fail to reach the junction among the carriersgenerated in the vicinity of the surface of the N type diffusion layer103 may increase and therefore cause degradation of the sensitivity ofthe light receiving device. In order to prevent such degradation of thesensitivity, a peak concentration in the impurity concentration profileshould preferably be placed on the surface of the N type diffusion layer103.

By setting the layer thickness of the N type diffusion layer 103 , i.e.,the junction depth at about 0.8 μm to 1.0 μm, further excellent responsespeed is obtained. FIG. 3 is a view showing changes in cathoderesistance when the junction depth is changed, where the horizontal axisrepresents a junction depth (μm) while the vertical axis representscathode resistance (Ω/sq.). As shown in FIG. 3, when the impurityconcentration in the vicinity of the surface of the N type diffusionlayer 103 is 1E19 cm⁻³, setting the junction depth at about 0.8 μm to1.0 μm enables sheet resistance of the N type diffusion layer 103 to benot larger than 200 Ω/sq. FIG. 4 is a view showing changes in responsespeed when the cathode resistance is changed, where the horizontal axisrepresents cathode resistance (Ω/sq.) while the vertical axis representsresponse speed represented by frequency (MHz). As shown in FIG. 4,setting the cathode resistance to not larger than 200 Ω/sq. enables thelight receiving device with a device area of about 70 μm×100 μm to havea response speed of not smaller than 1 GHz. Further, with an device areaof about 200 μm×200 μm, the response speed can be 500 MHz or more.Furthermore, in the case where the impurity concentration in thevicinity of the surface is about 5E18 cm ⁻³, a response speed equal tothat in the case where the impurity concentration is 1E19 cm⁻³ can beobtained by setting the junction depth at about 1.0 μm to 1.2 μm. FIG. 5is an example of an impurity concentration profile to be formed for theN type diffusion layer 103 when the surface concentration thereof is1E19 cm⁻³. In FIG. 5, the horizontal axis represents a depth (μm) fromthe surface of the light receiving part while the vertical axisrepresents impurity concentration (cm⁻³). For comparison with the PNjunction depth, the adsorption length of light with wavelength of 400 nmis also superimposed.

The above-configured light receiving device operates as follows. Thatis, once the light receiving device receives light, the light transmitsthrough the light transmitting film 104 and comes incident to the N typediffusion layer 103 almost without being reflected by the surface of theN type diffusion layer 103. When the light comes incident to the N typediffusion layer 103, carriers are generated. Since the N type diffusionlayer 103 is so formed that the impurity concentration on the surface is1E19 cm⁻³ and the impurity concentration is peaked on the surface,recombination of the carriers in the vicinity of the surface of the Ntype diffusion layer 103 is mostly prevented. Consequently, most of thecarriers reach the junction between the P type semiconductor layer 102and the N type diffusion layer 103. As a result, the light receivingdevice obtains excellent sensitivity. Moreover, since the N typediffusion layer 103 has a thickness of 0.8 μm to 1.0 μm, and thereforeits resistance is relatively low, the light receiving device is providedwith more excellent frequency response and implements high-speedoperation. More specifically, the light receiving device of the presentembodiment achieves both enhanced sensitivity and increased speed. Thelight receiving device is particularly suitable for receiving light withshort wavelength of 600 nm or less. Further, it is not necessary toadditionally provide a high-concentration diffusion layer for reducingresistance as with the conventional case, so that it is possible to makethe area of the light receiving part small and lessen a size of thelight receiving device.

In the above embodiment, an impurity used in the N type diffusion layer103 is not limited to phosphorous as long as it is pentavalent.

Further in the above embodiment, P type and N type conductivity may beinterchanged to each other.

Furthermore, as shown in FIG. 6, a plurality of cathode electrodes 108may be provided for reducing the resistance. The light receiving devicemay be a split type light receiving device having a plurality of lightreceiving parts. In this case, no restriction is placed on the form, thenumber or the forming method of the light receiving parts.

Further, the impurity concentration and the layer thickness of the Ptype diffusion layer 101 and the P type semiconductor layer 102 are notlimited to those disclosed in the present embodiment. It is alsopossible to delete the P type diffusion layer 101 and the P typesemiconductor layer 102, and instead, an N type diffusion layer isdirectly formed on the P type substrate 100 to form the PN junction.

(SECOND EMBODIMENT)

FIG. 7 is a graph showing an impurity concentration profile of an N typediffusion layer as a second conductivity type semiconductor layer and aP type semiconductor layer as a first conductivity type semiconductorlayer of a light receiving device in the second embodiment of thepresent invention. The N type diffusion layer forms a light receivingpart while the P type semiconductor layer contacts the N type diffusionlayer. In the N type diffusion layer that forms the light receivingpart, As (arsenic) is used as an impurity. The concentration profileshown in FIG. 7 is formed by detecting the impurity concentration withSIMS (Secondary Ion Mass Spectrometer).

The light receiving device in the second embodiment has the sameconfiguration as the light receiving device in the first embodimentexcept the point that the impurity of the N type diffusion is As. Inthis embodiment, description is made with use of the reference numeralsidentical to those used for the light receiving device in the firstembodiment shown in FIGS. 1A and 1B.

The light receiving device of the present invention has an impurityconcentration profile in which the concentration is not larger than1E19cm⁻³ in a depth of approximately the same length as the adsorptionlength of incident light from the top surface of the N type diffusionlayer 103. In this embodiment, the incident light has wavelength of 400nm, the thickness of the N type diffusion layer 103 , i.e., the junctiondepth is 0.8 μm, and the surface impurity concentration of the N typediffusion layer 103 is 1E20cm⁻³. The light receiving device of thepresent embodiment also has a peak impurity concentration in thevicinity of the surface like the first embodiment.

FIG. 8 is a graph showing changes in sensitivity of the light receivingdevice when impurity concentration in the vicinity of the surface of anN type diffusion layer 103, that is a cathode in a light receiving part.In FIG. 8, the horizontal axis represents surface concentration (cm⁻³)while the vertical axis represents sensitivity (A/W). As shown in FIG.8, when the impurity concentration in the vicinity of the surface of theN type diffusion layer 103 is about 1E20 cm⁻³ or less, the lightreceiving device obtains an excellent sensitivity characteristic. Here,it is not necessary to make a junction position between the N typediffusion layer 103 and the P type semiconductor layer 102 shallow likethe conventional case, and therefore the junction position is deepenedso as to reduce sheet resistance. This makes it possible to provide alight receiving device which can achieve both enhanced sensitivity andresponse. FIG. 9 is a graph showing changes in cathode resistance when ajunction depth is changed. In FIG. 9, the horizontal axis represents ajunction depth (μm) while the vertical axis represents cathoderesistance Ω/sq.). As shown in FIG. 9, when the junction depth is about0.8 μm, the sheet resistance or the cathode resistance of the N typediffusion layer 103 is about 50Ω/sq., which enables the response speedof the light receiving device to be not smaller than 1GHz. Morespecifically, even if the impurity concentration in the vicinity of thesurface of the N type diffusion layer 103 is increased to about 1E20cm⁻³ for reducing resistance, excellent sensitivity as well as higherspeed can be obtained by setting the concentration at a position fromthe surface of the N type diffusion layer 103 almost equal to theabsorption length of incident light to not larger than 1E19 cm⁻³.Particularly, the light receiving device in the present embodimentachieves effective enhancement of both the sensitivity and the responsespeed when it receives light with short wavelength of 600 nm or less.

In the above embodiment, although arsenic is used as an impurity in theN type diffusion layer 103, other pentavalent impurities may beacceptable if the profile same as shown in FIG. 7 is still formed.

(THIRD EMBODIMENT)

FIG. 10 is a cross sectional view showing a light receiving device in athird embodiment of the present invention. It is noted that in thepresent embodiment, multilevel interconnections and interlayer filmsformed after the step for processing metal interconnections are deleted.

The light receiving device of the present embodiment has a P typediffusion layer 201 with impurity concentration of about 1E18 cm⁻³ and athickness of about 1 μm formed on a P type silicon substrate 200, andhas a P type semiconductor layer 202 as a first conductivity typesemiconductor layer with impurity concentration of about 1E13 cm⁻³ to1E15 cm⁻³ and a thickness of about 10 μm to 20 μm formed on the P typediffusion layer 201. Reference numeral 203 is an N type semiconductorlayer. Reference numeral 204 is an N type diffusion layer as a secondconductivity type semiconductor layer with an impurity diffused forreducing resistance, and the impurity concentration in the vicinity ofthe surface is set to about 1E18 cm⁻³ to 1E20 cm⁻³ while a thicknessthereof is set at about 1 μm to 2 μm. It is to be noted that when theimpurity concentration in the vicinity of the surface of the N typediffusion layer 204 is set to not smaller than 1E19 cm⁻³, the impurityconcentration at a depth almost equal to the absorption length of thewavelength of incident light is set to not larger than 1E19 cm⁻³. Inthis case, an impurity of the N type diffusion layer 204 may be anyelement such as P, As and Sb (antimony) as long as the element ispentavalent. Further, the N type diffusion layer 204 preferably has animpurity concentration peak on the surface of the N type diffusion layer204. Reference numeral 205 denotes a light transmitting film as anantireflection film, which is composed of a silicon oxide 206 and asilicon nitride 207 as with the first embodiment. The N typesemiconductor layer 203 and the P type semiconductor layer 202constitute an NP junction. Reference numeral 208 is a P type diffusionlayer for pulling out an electrode from the anode.

FIG. 11 is a graph showing an impurity concentration profile of the Ntype diffusion layer 204, the N type semiconductor layer 203, and partof the P type semiconductor layer 202 in the above light receivingdevice. This impurity concentration profile is the one that can mosteffectively improve sensitivity and response speed even when light withwavelength of 400 nm is received. Although the light receiving devicehaving this impurity concentration profile has a very large junctiondepth of about 2.0 μm, excellent sensitivity is still achievable becausethe impurity concentration at a depth almost identical to the absorptionlength of incident light is not larger than 1E19 cm⁻³. Further, theresistance is as low as 50 Ω/sq., by which excellent response speed isalso attainable in this embodiment. The light receiving device in thisembodiment can enhance sensitivity and response speed particularly whenreceiving light with short wavelength of 600 nm or less.

(FOURTH EMBODIMENT)

FIG. 12 is a cross sectional view showing a light receiving unitincorporating a circuit in a fourth embodiment of the present invention.In the light receiving unit incorporating a circuit, a light receivingdevice D of the present invention and a bipolar transistor T as a signalprocessing circuit for processing a signal from the light receivingdevice D are formed on the same semiconductor substrate. In the presentembodiment, multilevel interconnections and interlayer films formedafter the step for processing metal interconnections are deleted.

The light receiving unit incorporating a circuit in the presentembodiment has a P type diffusion layer 301 with a thickness of about 1to 2 μm and impurity concentration of about 1E18 to 1E19 cm⁻³ formed ona P type silicon substrate 300 with impurity concentration of about 1E15cm⁻³. On this P type diffusion layer 301, there is formed a first P typesemiconductor layer 302 with a thickness of about 10 μm to 16 μm andimpurity concentration of 1E13 to 1E14 cm⁻³. On this first P typesemiconductor layer 302, there is formed a second P type semiconductorlayer 303 with a thickness of about 1 to 2 μm and impurity concentrationof about 1E13 to 1E14 cm⁻³. On this second P type semiconductor layer303, there is formed a LOCOS region 304 for separating the device.

In the light receiving device D part of the light receiving unitincorporating a circuit, an N type diffusion layer 305 as a secondconductivity type semiconductor layer with impurity concentration ofabout 1E18 to 1E20 cm⁻³ and a thickness of about 0.8 to 1.2 μm is formedin the second P type semiconductor layer 303 as a first conductivitytype semiconductor layer. The N type diffusion layer 305 constitutes acathode of the light receiving device. An impurity of the N typediffusion layer 305 may be any element such as P, As and Sb as long asthe element is pentavalent. This impurity forms an impurityconcentration profile as with the case of the light receiving devices inthe first and the second embodiments. By this, both increased speed andenhanced sensitivity of the light receiving device D is achieved.

Further, at least in a region on the second P type semiconductor layer303 to which light is radiated, there is provided a light transmittingfilm 306 as an antireflection film. This light transmitting film 306 iscomposed of a silicon oxide 307 with a thickness of 16 nm and a siliconnitride 308 with a thickness of about 30 nm, each disposed in this orderfrom the side of the second P type semiconductor layer 303.

Further, there is provided a second P type diffusion layer 309 extendingin a thickness direction from the top surface of the second P typesemiconductor layer 303 to the surface of the first P type diffusionlayer 301 through the second P type semiconductor layer 303 and thefirst P type semiconductor layer 302. The second P type diffusion layer309 is formed from B (boron) with concentration of about 1E18 to 1E19cm⁻³. Through this second P type diffusion layer 309, interconnectionsformed on the surface of the light receiving unit incorporating acircuit are electrically connected to the first P type diffusion layer301.

In the transistor T part of the light receiving unit incorporating acircuit, an N type well structure 310 made of P (phosphorus) withconcentration of about 1E17 to 1E19 cm⁻³ is formed in the second P typesemiconductor layer 303. In order to reduce resistance of this N typewell structure 310, an N type diffusion layer 311 made of P (phosphorus)with concentration of about 1E18 to 1E19 cm⁻³ is provided below the Ntype well structure 310. In part of the region of the N type wellstructure 310, there is formed an N type diffusion layer 312 made ofphosphorus with concentration of about 1E19 to 2E19 cm⁻³ serving as acollector contact of the transistor. Further, in part of the region ofthe N type well structure 310, there are formed a P type diffusion layer313 made of B (boron) with concentration of about 1E17 to 1E19 cm⁻³serving as a base of the transistor and an N type diffusion layer 314made of As serving as an emitter.

Further, there are formed a cathode electrode (unshown) for pulling outan electrode from the N type diffusion layer 305 of the light receivingdevice D, an anode electrode 315 connected to the P type diffusion layer309, as well as a collector electrode 316, a base electrode 317, and anemitter electrode 318 of the transistor.

The above-configured light receiving unit incorporating a circuit hasthe light receiving device D which can effectively achieve both asensitivity characteristic and a response characteristic, and isparticularly suitable for receiving light with short wavelength.

In the above embodiment, although an NPN transistor is used, a PNPtransistor or the both transistors may be formed on the substrate.

Further, without being limited to the configuration defined in thepresent invention, the transistor T may employ other configurations.

Furthermore, a signal processing circuit formed on the silicon substrate300 together with the light receiving device may be a MOS(Metal-Oxide-Semiconductor) transistor and a BiCMOS (Bipolar CMOS) otherthan the bipolar transistor.

(FIFTH EMBODIMENT)

FIG. 13 is a view showing an optical pickup provided in the optical diskdrive in the fifth embodiment of the present invention. The opticalpickup splits light with wavelength of approx. 400 nm emitted by asemiconductor laser 400 into three beams, consisting of two side beamsfor tracking and one main beam for reading signals, with use of adiffraction grating 401 for generating tracking beams. After transmittedthrough a hologram 402 as zero-order light and converted to parallellight by a collimate lens 403, these beams are collected on a disk 405by an object lens 404. The light collected on the disk 405 is reflectedwith light power being modulated by a pit formed on the disk 405, andafter this reflected light transmits through an object lens 404 and thecollimate lens 403, the light is diffracted by the hologram 402. Afirst-order optic component diffracted by the hologram 402 comesincident to a split type light receiving device 406 having five lightreceiving faces from D1 to D5. Then, calculating the outputs from thesefive light receiving faces by adding to and subtracting from to eachother, a reading signal and a tracking signal are obtained.

The split type light receiving device 406 is a light receiving device ofthe present invention, and the N type semiconductor layer as a secondconductivity type semiconductor layer forming the five light receivingfaces has a thickness, i.e., a junction depth larger than the absorptionlength of the wavelength of incident light from the hologram 402, andhas an impurity concentration profile similar to that in FIG. 7.Therefore, the split type light receiving device 406 has impurityconcentration at a depth almost equal to the absorption length of theincident light of not larger than 1E19 cm⁻³, which provides excellentsensitivity. Further, the split type light receiving device 406 has ajunction depth larger than the absorption length of incident light, sothat the resistance is as low as about 50 Ω/sq., by which excellentresponse speed is attained. Therefore, the split type light receivingdevice 406 has excellent sensitivity and response speed, so that theoptical pickup is suitable for read and write access to high-densityoptical disks.

In the present embodiment, the optical pickup may adopt other opticalsystems other than the optical system shown in FIG. 13.

Further, the semiconductor laser 400 may emit light with wavelengthother than that of about 400 nm.

1. A light receiving device comprising: a first conductivity typesemiconductor layer; and a second conductivity type semiconductor layeron the first conductivity type semiconductor layer, wherein a thicknessof the second conductivity type semiconductor layer is larger than anabsorption length of light to be received incident to the secondconductivity type semiconductor layer, and the second conductivity typesemiconductor layer has an impurity concentration of not smaller than1E17 cm⁻³ and not larger than 1E19 cm⁻³ in a vicinity of a surface ofthe second conductivity type semiconductor layer.
 2. A light receivingdevice as defined in claim 1, wherein: the second conductivity typesemiconductor layer has said impurity concentration at a position whichis at a distance from the surface of the second conductivity typesemiconductor layer in a thickness direction, said distance being almostequal to the absorption length of the light.
 3. The light receivingdevice as defined in claim 1, wherein the second conductivity typesemiconductor layer has a peak impurity concentration on the surface. 4.A light receiving unit incorporating a circuit comprising: a lightreceiving device, comprising: a first conductivity type semiconductorlayer; and a second conductivity type semiconductor layer on the firstconductivity type semiconductor layer, wherein a thickness of the secondconductivity type semiconductor layer is larger than an absorptionlength of light to be received incident to the second conductivity typesemiconductor layer, and the second conductivity type semiconductorlayer has an impurity concentration of not smaller than 1E17 cm⁻³ andnot larger than 1E19 cm⁻³ in a vicinity of a surface of the secondconductivity type semiconductor layer; and a signal processing circuitfor processing a signal from the light receiving device, wherein thelight receiving device and the signal processing circuit are formed onan identical substrate.
 5. An optical disk drive comprising: a lightreceiving device, the light receiving device comprising: a firstconductivity type semiconductor layer; and a second conductivity typesemiconductor layer on the first conductivity type semiconductor layer,wherein a thickness of the second conductivity type semiconductor layeris larger than an absorption length of light to be received incident tothe second conductivity type semiconductor layer, and the secondconductivity type semiconductor layer has an impurity concentration ofnot smaller than 1E17 cm⁻³ and not larger than 1E19 cm⁻³ in a vicinityof a surface of the second conductivity type semiconductor layer.
 6. Anoptical disk drive comprising: a light receiving unit incorporating acircuit comprising: a light receiving device, comprising: a firstconductivity type semiconductor layer; and a second conductivity typesemiconductor layer on the first conductivity type semiconductor layer,wherein a thickness of the second conductivity type semiconductor layeris larger than an absorption length of light to be received incident tothe second conductivity type semiconductor layer, and the secondconductivity type semiconductor layer has an impurity concentration ofnot smaller than 1E17 cm⁻³ and not larger than 1E19 cm⁻³ in a vicinityof a surface of the second conductivity type semiconductor layer; and asignal processing circuit for processing a signal from the lightreceiving device, wherein the light receiving device and the signalprocessing circuit are formed on an identical substrate.
 7. A lightreceiving device comprising: a first semiconductor layer having a firstconductivity type; a second semiconductor layer having a secondconductivity type formed on the first semiconductor layer; and a thirdsemiconductor layer having the second conductivity type formed on thesecond semiconductor layer, wherein a thickness of the thirdsemiconductor layer is larger than an absorption length of light to bereceived incident to the third semiconductor layer, and the thirdsemiconductor layer has an impurity concentration of not smaller than1E17 cm⁻³ and not larger than 1E19 cm⁻³ in a vicinity of a surface ofthe third semiconductor layer.
 8. The light receiving device as definedin claim 7, wherein the third semiconductor layer has said impurityconcentration at a position which is at a distance from the surface ofthe third semiconductor layer in a thickness direction, said distancebeing almost equal to the absorption length of the light.
 9. The lightreceiving device as defined in claim 7, wherein the third semiconductorlayer has a peak impurity concentration on the surface.
 10. A lightreceiving unit incorporating a circuit comprising: a light receivingdevice including: a first semiconductor layer having a firstconductivity type; a second semiconductor layer having a secondconductivity type formed on the first semiconductor layer; and a thirdsemiconductor layer having the second conductivity type formed on thesecond semiconductor layer, wherein a thickness of the thirdsemiconductor layer is larger than an absorption length of light to bereceived incident to the third semiconductor layer, and the thirdsemiconductor layer has an impurity concentration of not smaller than1E17 cm⁻³ and not larger than 1E19 cm⁻³ in a vicinity of a surface ofthe third semiconductor layer; and a signal processing circuit forprocessing a signal from the light receiving device, wherein the lightreceiving device and the signal processing circuit are formed on anidentical substrate.
 11. An optical disk drive comprising: a lightreceiving device including: a first semiconductor layer having a firstconductivity type; a second semiconductor layer having a secondconductivity type formed on the first semiconductor layer; and a thirdsemiconductor layer having the second conductivity type formed on thesecond semiconductor layer, wherein a thickness of the thirdsemiconductor layer is larger than an absorption length of light to bereceived incident to the third semiconductor layer, and the thirdsemiconductor layer has an impurity concentration of not smaller than1E17 cm⁻³ and not larger than 1E19 cm⁻³ in a vicinity of a surface ofthe third semiconductor layer.
 12. An optical disk drive comprising: alight receiving unit incorporating a circuit comprising: a lightreceiving device including: a first semiconductor layer having a firstconductivity type; a second semiconductor layer having a secondconductivity type formed on the first semiconductor layer; and a thirdsemiconductor layer having the second conductivity type formed on thesecond semiconductor layer, wherein a thickness of the thirdsemiconductor layer is larger than an absorption length of light to bereceived incident to the third semiconductor layer, and the thirdsemiconductor layer has an impurity concentration of not smaller than1E17 cm⁻³ and not larger than 1E19 cm⁻³ in a vicinity of a surface ofthe third semiconductor layer; and a signal processing circuit forprocessing a signal from the light receiving device, wherein the lightreceiving device and the signal processing circuit are formed on anidentical substrate.